Volume control



Sept. 27, 1932. H. A WHEELER 1,879,863

VOLUME CONTROL Original Filed July 7. 1927 4 Sheet's-Sheet l Sept. 27,'1932.

H. A. WHEELER VOLUME CONTROL Original Filed July 7, 1927 4 Sheets-Sheet2 @eier i UO r bw P F H n N n f n H n m ww NW .SW uw@ 4 Sheets-Sheet '5INVENTOR ATTORNEYS Harold waaier BY 'Hlm/2.4, .L4-ul NNY SePt- 27, l932-H. A. WHEELER VOLUME CONTROL Original Filed July 7, 1927 Sept. 27, 1932.

H. A. WHEELER VOLUME CONTROL 4 Sheets-Sheet 4 Original Filed` July 7,1927 SSB@ ATTORNEYS Patented Sept. 27, 1932 UNITED STATES PATENTOFI-'ICE HAROLD A. WHEELER, OF GREAT NECK, NEW YORK, A.ASSIGNIOR TOHAZELTINE CORPO- RATION, A CORPORATION OF DELAWARE VOLUME CONTROLOriginal application led July 7, 1927, Serial No. 203,879, and in GreatBritain July 3, 1928.

REISSUED Divided and this application led November 13, 1930. Serial No.495,386.

This invention relates to amplifiers, and more particularly toamplifiers utilized in modulated carrier-current signaling systemswherein the limit of amplification is automatically maintainedsubstantially at a predetermined level.

This application is a division of application Serial No. 203,879, filedJuly 7, 1927.

flien amplifiers are employed for amplifying a signal voltage it becomesdesirable for various reasons to control automatically the amplitude ofthis amplified signal voltage. To this end the present inventionprovides means for effecting automatic amplification control. Such anarrangement, for example, is particularly advantageous in radioreceivers such as are employed for receiving broadcast signals, becauselit prevents the overloading of the last amplifier stage of thereceiver, which overloading would result in distortion of the reproducedsignal, as well as loud and harsh reproduction.

Another advantage resides in uniform reproduction of the amplifiedsignal irrespective of whether the carrier-current signal is receivedfrom a nearby station or from a distant or a high-power station, or alowpower station, since it has been found in former radio receivers thatwhen the receiver was reproducing strong signals as from a nearby, or ahigh-power station, the audibly reproduced signal was very loud, whereaswhen the signal was received from a distant, or a low-power station, itwas relatively weak, with the result that if signals were to bereproduced uniformly from both near and distant stations, and fromhigh-power and low-power stations, it` became necessary to readjust somevolume controlling mea-ns in the receiver to compensate for theseunequal signals.

It has also been aI common experience in the use of former radioreceivers that the reproduced signal was not uniform due to thephenomenon of fading, whereby the received signal occasionally, orperiodically became much'weaker, or faded almost to the point ofinaudibility. Since the present invention provides an amplifier whichautomatically compensates for inequalities in the lpower system, it hasbeen found that received carrier-current signal strength, when fadingtakes place the degree of amplification is correspondingly increased andthe reproduced signal maintained at its former volume, so that alistener is unaware that variation of the received carrier-currentsignal is occurring. This automatic compensation for .signal fading isespecially advantageous in commercial radio telephony and like systems.

In existing radio receivers in which operating current is derived fromthe municipal when there is considerable variation in the line voltagesupply, the volume of the reproduced signal is not uniform. Anadditional advantage of the present invention is that of automaticallycompensating for such line voltage variations with the result that thereproduced signal is uniform in volume. v

A still further advantage is the saving in plate current which isautomatically effected during the reception of powerful signals, for thereason that this invention incidentally provides means for reducing theplate current of one or more amplifying tubes as the signal strengthincreases. y

Fig. lis a circuit diagram of a complete radio receiver which includesthe present invention, and consists of a three-stageradiofrequencyamplifier followed by a rectifier, a two-stageaudio-frequency amplifier, and a loud speaker, or other suitableindicating device.

Fig. 2 shows curves disclosing the relation between the radio-frequencyantenna voltage and the radio-frequency amplified voltage, with andwithout ent invention.

Fig. 3 shows a circuit diagram of a second embodiment of the inventionin which there is disclosed a three-stage tuned radio-frequencyamplifier, a rectifier, and a three-stage audio-frequency amplifier.

Fig. 4 represents the present invention embodied in a radio receiverincluding a twostage radio-frequency amplifier, a detector, and atwo-stage audio-frequency amplifier.

Fig. 5 shows a modified form of coupling arrangement andcontrolling'network which the application of the presmay be substitutedfor the correspondin elements inclosed in the dotted rectange ofbatteries now so widely used.

Fig. 7 is a circuit da am of a radio receiver in which several o thefeatures of the by a variable condenser 22 present invention arecombined in a single embodiment.

Fig. 8 shows graphically a comparison between the performance of thetwo-electrode valve or rectifier, and of the three-electrode detector. lReferring in detail to Fig. 1, there is shown an antenna 5 connected toground '1 through the primary winding 6 of a radio-frequencytransformer, the secondary winding 7 of which, tuned by a variablecondenser 8, is connected at one point to the filament of the vacuumtube 9 in the first radio-frequency amplifying stage and at anotherpoint to the grid 11 o this vacuum tube. The output circuit of thisvacuum tube extends from the filament system, through a high-voltagebattery B, a milliammeter 10, primary winding 13 of a secondradio-frequency transformer to the anode or plate 14 of this-vacuumtube. In order to neutralize the inherent capacity between the grid 11and the plate 14, and thereby to prevent oscillations, and otherwise toincrease the effectiveness of the present invention as hereinafterdescribed, a neutralizing winding 19, electromagnetically coupled towinding 13, and a neutralizing condenser 3 are employed in the mannerdescribed in the U. S. patents to Hazeltine Nos. 1,489,228 and1,533,858.

A second stage of radio-frequency amplification including the vacuumtube 15 neutralized by cooperation of coil 26 and condenser 4, like thefirst stage, comprises the secondary winding 16 of the last-mentionedradio-freuency transformer tuned by a variable condenser 17 connectedbetween the filament system of the vacuum tube 15 and the grid 18thereof. The output circuit of this vacuum tube also includes thehigh-voltage battery B and a primary winding20 of a secondradio-frequency transformer, while the secondary winding 21 of thistransformer tuned is included in the input circuit of a third stage ofradio-frequency amplification which includes vacuum tube 23. Theinherent capacity effective between the electrodes 24 and 25 isneutralized by a network including the neutralizing condenser 28 and theneutralizing winding 29 as described in the mentioned atents. The outputcircuit of the vacuum tu e 23 includes the primary winding 30 of a thirdradio-frequency transformer and the high-voltage battery B. Thesecondary winding 31 of this lastmentioned transformer, tuned by avariable condenser 32, is connected in the input circuit. of a rectifier33 which input circuit includes the -fixed condenser 2, the rectifieremployed may be of the type commonly known in the art as a two-electrodeFleming valve, or may consist of an e uivalent such as a threeelectrodevacuum tu e, as shown, having its grid 12 and its plate or anode 35directly connected together to comprise in effect a single anode.

It may here be noted that throufrhout the present specification andclaims the terms rectifier and detector aref, in general, usedinterchangeably, the terms rectifyng and converting being employed inthe general sense to include the process of changing alternatin gcurrent into a form of direct current or modulated unidirectionalcurrent. Likewise, the terms carrier-current and modulation current maybe substituted, respectively, for radio-frequency current andaudiofrequency current, since the description herein of radio-frequencyamplifiers and audio-frequency amplifiers is merely by way of example ofa typical embodiment of the present invention.

In the absence of the present invention including the control circuit36, to be described, the three-stage amplifier functions in a mannerwell-known in the art to amplify the incoming signal intercepted on theantenna 5. The output circuit of the rectifier 33 includes what may betermed a rejector circuit for stopping radio-frequency currents whichhave passed through the rectifier, and consists of a network including aresistance 34 and a by-pass condenser 37 connected between the anode 35and the filament 38 of the rectifier. The output circuit of therectifier is coupled to the input circuit of an audio-frequencyamplifying vacuum tube 39 through an audio-frequency-pass filterincluding a fixed condenser 40 and an impedance 41 connected between thefilament 42 and the grid 43 of this vacuum tube. The output circuit ofthis amplifier is connected between the filament 42 and plate 44 throughthe highvoltage battery B and the primary winding 45 ofanaudio-frequency transformer, the secondary winding 46 of which isconnected in the input circuit of a second audio-frequency tube 47,while a resistance 48 connected across the winding 46 serves to give theaudio amplifier substantially uniform amplification over the desired.frequency range. Instead of employing resistance 48, a closed copperband of suitable size may be placed around the transformer winding so asto be electromagnetically coupled thereto. A loud speaker or otherreproducing device 50, or if required, a coupling device for a telephonesystem, is connected in the output circuit of the last audio-frequencyamplifying tube 47. It is presumed that adequate precautions againstundesired electromagnetic coupling between the various radio-frequencycoupling transformers are included in all of the arrangements hereindisclosed.

In accordance with the main feature of the present invention, means areprovided to control automatically the degree of amplification effectedin the radio-frequency amplifying stages. These means include aresistance 51, connected between the filament 38 and the anode 35 of therectifier, through which the I pulsating rectified or converted currentfiows,

thereby developing a negative voltage at terminal 52. This negativevoltage is applied over conductor 3G through the impedance 53 and thesecondary winding 7 of the first radio-frequency transformer to grid 11of the rst radio-frequency stage. Impedance 53,

together with blocking condenser 54, is effective to filter out andreject any audio-frequency currents which otherwise might be present inthe conductor 36.

To complete the description of the system illustrated in Fig. l certaindesign data or constants are given herewith. It should be understood,however, that these, as well as all other constants appearing in thepresent specification, are mentioned merely by way of example indescribing certain specific embodiments which in practice have provedeminently satisfactory, and are not intended to suggest any specificlimitations as to the scope of this invention. Accordingly, fixedcondenser 2 may be of 0.0005 microfarads; 37 of 0.0001 microfarads; 54of 0.01 microfarads; 40 of 0.005 microfarads; resistance 51 of 1 megohm;34 of 1 megohm; and 41 and 53 of 2 megohms each.

In the operation of the receiver shown in Fig. 1 a signal intercepted onthe antenna 5 is `successively amplified through the neutralizedradio-frequency stages indicated by the vacuum tubes 9, 15 and 23. Thisamplified signal voltage is then rectified by the rectifier 33, and theVrectified pulsating current is successively amplified by the audioamplifying stages including vacuum tubes 39 and 47, after which it maybe reproduced as sound by the loud speaker 50. When the rectified orconverted signal current fiowing through the resistance 51 is greaterthan-a predetermined value. there is developed at the termin al 52sufficient negative biasing voltage which in turn is impressed, throughthe conductor 36, upon the grid 11 ofthe vacuum tube 9. to reduce theamplification of this tube. It will be apparent that as the magnitude ofthe rectified current flowing through resistance 51 decreases withweaker signals, the voltage at terminal 52 becomes less negative, andthe negative biasing voltage impressed upon the lgrid 11 also diminishesso that the vacuum tube 9 effects an increased degree of amplification.In this manner, the radio-frequency voltage applied to the input of therectifier is maintained at a nearly constant predetermined value, andthe .volume of the reproduced 'signal is substantially uniform under allconditions. The degree of volume of the reproduced signal is thendetermined by adjustment of rheostat 49 which controls the heatingcurrent in the filament 42 of the first audio-frequency amplifying tube39. The neutralization of the gridplate capacity of the radio-frequencyamplifying tubes is, in combination with the present invention,particularly valuable in that it allows an increase in the effectivenessof the amplification control, because such neutralization preventsradio-frequency energy from passing through the grid-plate capacity ofthe tubes. Thus the relay action of the tubes is almost entirely subjectto the control by grid bias voltage provided in accordance with thisinvention.

The time required for operation of the control system would ordinarilybe determined by the lowest audio-frequency modulation which must bereproduced. Fading, for example, might be considered a form ofmodulation; the frequency of the rise and fall of signals due to fadingbeing the frequency of modulation. If this frequency of modulation beincreased sufficiently, the effect will be audio-frequency modulation.It will thus be seen that if the automatic control attained by thepresent invention be allowed to respond too quickly, it will tend tosmooth out the desired modulation ofthe signals at the lower audiofrequencies. Hence, a time constant of operation is chosen which will begreater than the period of the audio frequencies which the system isintended to amplify. This time constant of the control circuit is equalto the product of the series resistance and the shunt capacitance of thegrid bias circuit, represented in Fig. 1 by resistance 53 andcapacitance 54. However, since the time constant can always be reducedto a value equal to the period of the lowest modulation frequency, itmay readily be set to meet the requirements of nearly any special casewhich may arise. For example, a value of two million ohms resistance andof 0.1 microfarad capacitance gives a time constant of one-fifth of asecond, which does not appreciably affect the modulation of frequenciesabove five cycles. While this constant is greater than required from thepoint of view of satisfactory audio-frequency quality in thereproduction of music, there appears to be no need for more rapidcontrol under the conditions usually encountered. The use in thisconnection of condensers of large capacitance, such as one-tenthmicrofarad, likewise introduces another convenience in that thecondensers may also serve to by-pass radio frequencies in order toprevent undesired coupling between the detector circuit and the firstradio-frequency amplifying tube because of some impedances common tothose two portions of the appadio-frequency voltage is indicated bycurve 103 from which it will be seen that when at least a certainpredetermined radio-frequency antenna voltage is present, (hereinreferred to as the threshold antenna voltage) the amplifiedradio-frequency voltage approaches-but is always less than-anothercertain predetermined voltage value (herein referred to as the cut-offvoltage).

The modification illustrated in Fig. 3 is an especially desirable formof the present invention, and includes antenna 56, connected by means ofa transformer 57 to a neutralized three stage tuned radio-frequencycascade amplifier including the vacuum tubes 58, 60 and 62coupled bytransformers 59 and 61. The last stage of the amplifier is connected bya transformer 63 to a two-electrode rectifier 64 of the type alreadydescribed, the output circuit of which, including the resistance 65, isconnected between the anode 66 and filament 67 of the rectifier, aspreviously explained. Resistance 72 and condenser 68 associated withthis output circuit, constitute a rej ector network which filters outthe radio-frequency current component in the output circuit of therectifier 64, While the network including condenser 69 and resistance 70constitutes an audio-frequency-pass filter for coupling the outputcircuit of the rectifier to the input circuit of the audio-frequencyamplifier which includes vacuum tube 71. Rheostat 7 3 controls theheating current supplied to the filament 74 of this vacuum tube, andthereby permits a manual adjustment of the volume of the reproducedsignal desired by thelistener. Audio-frequency transformer 76, which ispreferably of a low ratio of transformation, couples the output circuitof vacuum tube 71 to a second audio-frequency amplifying tube 77. Thislast vacuum tube in turn is coupled by a second audio-frequencytransformer 78 to a third audio-frequency amplifying tube 79 in theoutput circuit of which there is included a loud speaker 80.

In this arrangement automatic amplification control is effected in amanner slightly different from that shown in the diagram of Fig.' 1,since in this instance the radiofrequency voltage of the signalsintercepted by the antenna 56 is successively amplified by threeneutralized tuned radio-frequency amplifying stages including the vacuumtubes 58, 60 and 62, of which two are controlled in accordance with thepresent invention. The amplified radio-frequency current is rectified bythe rectifying valve 64, and successively amplified at audio-frequencyby the vacuum tubes 71, 77 and 79. The rectified current in the outputcircuit of the rectifier flows through the resistance 65, and therebydevelops a negative-voltage at the terminal 81, which voltage is appliedthrough the impedances 72, 82, 83 and 85 to the rids 84 and 86 of theradio-frequency ampli ying tubes 58 and 60. By thus simultaneouslycontrolling the degree of amplification of two successiveradio-frequency amplifying stages agreatly increased uniformity ofregulation is attained. Impedance 82 and the condenser 87 constitute anaudio-frequency-stop filter, so that substantially only direct-voltageis impressed upon the grids 84 and 86. It will be understood that thevoltage developed at terminal 81 is a function of the amplifiedradio-frequency voltage delivered to the input circuit of the rectifierby the radio-frequency amplifying tubes 58, 60 and 62, and therefore, asthe negative voltage at terminal 81 tends to increase with the increasedsignal, the resulting increase of biasing voltage impressed upon thegrids of the tubes 58 and 60 limits the degree of amplification effectedin the radio-frequency stages including those tubes.

In this arrangement the constants for the varlous resistances andcondensers may, for example, be the same as those for the correspondingelements in Fig. 1. In addition the grid resistances 83 and 85 may havea value of 2 megohms each; and the grid condensers connected at thejunction of these resistances and the grid electrodes 84 and 86 may eachbe of 0.001 microfarad capacity.

The modification shown in Fig. 4 differs from the arrangement of Figs. 1and 3 mainly in that it employs a. three-electrode vacuum tube whichfunctions in the manner of the Well-known three-electrode detector, andalso effects rectification, or conversion, of the radio-frequencycarrier current to control amplification in the first radio-frequencystage of the receiver. As in the preceding arrangements, there is hereemployed an antenna or other suitable signal interceptor 88 coupled bymeans of a radio-frequency-transformer 89 to a two-stage neutralizedtuned radio-frequency amplifier including the vacuum tubes 90 and 91coupled by means of a radio-frequency transformer 92. The output circuitof the last stage of the amplifier is connected by means ofradio-frequency transformer 93 to the tuned input circuit of athree-electrode vacuum tube detector 94, which inputk circuit is tunedby the inductance of the secondary winding of transion former 93 inshunt to variable condenser 95. A suitable negative voltage ismaintained on the grid 96 of the detector tube, through the secondaryWinding of transformer 93, by C battery 97 and by potentiometer 98connected across the filament of the detector tube. By means of thispotentiometer connection, a negative voltage may be applied to the grid96 varying from one volt to a maximum of five volts; the minimum valuebeing the difference between the six Volts of C battery 97 and thevoltage drop across the filament of the detector. The output circuit ofthe detector includes the primary winding of transformer 99, a 45-voltbattery 100 and a 500,000 ohm resistor 101, connected in series betweenthe anode 0f the detector and the common B battery. A fixed condenser104 by-passes the radio-frequency current that has passed through thedetector, while the audio-frequency component of the rectified, orconverted, current is transferred through the audio-frequencytransformer 99 to the input circuit of an audio-frequency amplifyingvacuum tube 105, the filament circuit of which includes rheostat 106 forcontrolling the arbitrary volume level of the amplified signals. Theoutput circuit of the audio-frequency amplifier 105 is coupled by meansof an audio-frequency transformer 107 to the input circuit of a secondaudiofrequency amplifying tube 108. The output circuit of this vacuumtube includes the usual loud speaker or indicating device 109. Aresistance 110 is connected across the secondary winding of transformer107 to secure substantially constant amplification within the frequencyrange of the audio-frequency amplifier, especially when the plateresistance of the preceding tube 105 is high as a result of theadjustment of rheostat 106.

For controlling the amplitude of the radiofrequency voltage applied tothe input circuit of detector 94, a conductor 111 is con' nected atpoint 112 common to a terminal of resistor 101 and battery 100, andthence through the secondary winding of transformer 89 to the grid ofthe first radio-frequency amplifying tube 90. A by-pass condenser 113connecting the conductor 111 to the filament system serves to filter outand reject any audio-frequency currents present in the circuit includingthe conductor 111, thereby insuring that these currents have no effectonthe grid of the vacuum tube 90. Battery 1 00 is the source of negativebiasing voltage applied to the control grid, or controlelectrode, of theradio-frequency amplifying tube, this battery being so connected to theoutput circuit of the detector, or rectifier, that fluctuations ofvoltage in the detector output circuit cause equal fluctuations in thenegative biasing voltage impressed upon the control-grid.

In this embodiment, condensers 104 and 113 may be of 0.0005 microfaradand 1 micrfarad capacityrespectively; while resistance 101 may have avalue of 0.5 megohm.

In adjusting the receiver of Fig. 4, it is necessary to determine thecorrect setting of the detector grid potentiometer 98. This adjustmentshould be made while there is no signal being received, as follows:First, the switch 115 is closed, and the normal plate current of thetube 90 is noted on milliammeter 116. Then the switch is opened, thusplacing the control circuit in operation. In general, the plate currentof vacuum tube 90 will change when the switch is opened, since the gridvoltage of this tube is dependent upon the cont-rol circuit. By varyingthe grid voltage of the detector by potentiometer 98. the plate currentof tube 90 is then adjusted to the normal value; and the apparatus isready for operation. Upon receiptV of an amplified signal at thedetector, the effect of the control circuit is to decrease the platecurrent through milliammeter 116, thereby reducing the amplification ofthe tube 90. When the receiver is tuned to e signal frequency, a minimumamplificatio is required, so that when the condition of sonance isattained, the plate current of t be 90 is at a minimum value.

It is believed unnecessary to explain the operation of the tworadio-frequency amplification stages and of the detector, or of the twoaudio-frequency amplification stages, for

inoperation they are substantially similar to those of the nowwell-known type of radio receiver employing neutralized two-stageradio-frequency and two-stage audio-frequency amplifiers. Thecontrol'circuit operates, in the arrangement of Fig. 4, substantially inthe same manner as in Figs. 1 and 3, to apply a negative biasing voltageto the grid of the radio-frequency amplifying vacuum tube 90, thisvoltage being a function of the radio-frequency voltage which has beenamplified by the vacuum tubes and 91 and then applied to the inputcircuit of detector 94.. Since the voltage applied over conductor 111 isa function of the amplified radio-frequency voltage, there is a maximum,or cut-oli", detector voltage determined b the constants of the circuit,as shown in Fig'. 2, beyond which the radio-frequency amplifier isprevented from effecting further amplification. This arrangementmaintains the finally-amplified radio-frequency voltage at substantiallyconstant value.

Fig. 5 shows an alternative system for coupling the detector to thefirst audio-frequency amplifying tube of Fig. 4. The couplingarrangement of Fig. 5 included within the dot-ted rectangle, whensubstitutedfor the corresponding portion enclosed within the rectangleof Fig. 4, provides a modified form of the invention. Correspondingelements of Athese two figures are identified by the same referencecharacters, from which it will be seen that Fig. 5 differs from Fig. 4in that the transformer coupling between the detector 94 and the firstaudio-frequency amplifying tube 105 has been replaced by an impedancecoupling arrangement including the condenser 117 and the impedance 118.

While this modification does not utilize a transformer having a step-upratio such as is included ink the arrangement of the former figure, it,nevertheless, introduces the advantage of effecting a more nearlyconstant degree of amplification at audio frequencies. When themodicationl of Fig. 5 is substituted in the system of Fig. 4, asdescribed, the values of elements 104 and 101 may be the same asmentioned above; the resistance connected in lead 111 may be of 2megohms; 1'18 of 2 megohms; 117 of 0.005 microfarad; and 113 of 0.5microfarad.

Referring to Fig. 6, there is shown a radio receiver of theunneutralized type in which the so-called A, B and C batteries have beenreplaced by a source of rectified and filtered alternating current. Inthis embodiment of the invention there are provided three stages oftuned radio-frequency amplification in which the vacuum tubes of thesuccessive stages are designated 119, 120 and 121, respectively. Theseseveral stages are transformer-coupled; and the last stage of theamplifier is coupled to a three-electrode vacuum tube detector 122, thegrid bias voltage of which is controlled by potentiometer 128. In theoutput circuit of detector 122, there is provided a rejector circuit,similar to that previously described, for filtering out radio-frequencycurrents that have passed through the detector; and also anaudiofrequency network, or impedance coupling, including condenser 123and impedance 124, for passing the audio-frequency component of therectified signal to the first audio-frequency amplifying vacuum tube125. The filament of this tube is shunted by rheostat 129 whichfunctions as a manual volume control. This last tube istransformer-coupled to a second stage of audiofrequency amplification,including the vacuum tube 126, in the output circuit of which there isprovided a loud speaker, or other suitable indicating device, 127, whichon occasion may be replaced by a coupling device in a telephone system.The filaments of these siX vacuum tubes are connected in seriesv acrossa suitable resistance in the rectified, filtered source of power supply,giving a potential difference of 30`volts, thus taking the place of an Abattery. The necessary C, or bias, voltage is derived from a potentialdifference across a. resistance in that portion of the power supplyindicated by the reference character C, while theplate-current supply issimilarly derived from a resistance in that portion of the power supplyindicat- Lenses ed by the reference-letter, B. It should be noted thatthe filament of the first radiofrequency amplifying tube 119 isconnected to the positive terminal of the 30-volt A section of the powersource, and that' the filament of the detector tube 122 is connected tothe negative terminal of this section. The constants 'of the elements ofthis embodiment may in general be similar to those suggested withreference to Fig. 5.

In adjusting the receiver of Fig. 6, the potentiometer 128 is adjustedwith switch 115 open, as described in connection with the adjustment ofthe receiver of Fig. 4. This arrangement with thevacuum tube filamentsconnected in series obviates the necessity of a separate batterycorresponding to 100 of Figs. 4 and 5, since the plate of the detectortube 122 can be positive relative to the filament of .that tube, and atthe same .time maintains the grid of the first radio-frequencyamplifying tube 119 negative relative to the filament of the same tube,due to the difference of potential between the two filaments. Thus thebiasing voltage applied to the control-grid is derived from the voltageacross the filaments instead of from a battery as in Figs. 4 and 5.

n Figs. 4 and 5, batteries B and 100 are connected in series in theplate circuit of detector 94 and both contribute to the detector platecurrent. The B battery supplies the voltage drop in resistor 101, whilebattery l 100 supplies the plate voltage of detector 94. The presence inthe detector plate circuit of the B battery, which is directly connectedto the grounded filament circuit, in addition to auxiliary battery 100,allows the use of a high impedance 101 in the detector plate circuitWith a resulting high sensitivity of the detector circuit. The sameresult is achieved in the arrangement of Fig. 6 by the cooperation ofthe A and B voltages in the detector plate circuit, as described before.In the event that tubes having an indirectly heated cathode are usedinstead of those having an incandescent filament cathode as representedin the figure, the same advantages may be obtained as pointed out inconnection with Fig. 6, if the detector cathode is maintained at apotential much more negative than the cathode of the controlled tube ortubes, which, in the ure, is the first radiofrequency amplifier tu e.

The circuit arrangement shown in Fig. 7 incorporates several advantagesintroduced by the present invention, some of which have beenindividually described above. Briefly,

this arrangement includes a combination of the features illustrated inand described in connection with Fig. 3 and Fig. 6. The referencecharacters of-Fig. 7 correspond to l those employed in Fig. 3 and havethe same significance. It will be noted in addition to the apparatusrepresented in Flg. 3, that lli there is here illustrated a power sourceof rectified and filtered alternating current which replaces the`so-called A and B batteries represented in Fig. 3. and in addition,includes a source of 0, 0r grid bias voltage for tube 79. The grid oftube 7 7 is biased by connecting the grid return lead to an appropriatepoint in the series filament circuit, as shown. The power source issimilar to that shown in and described in connection with Fig. 6. Thepresent arrangement thus includes the advantages of neutralizedradiofrequency amplifying stages. automatic volume control applied tothe first two stages of the radio-frequency amplifier, a twoelectrodevalve, or rectifier, and the complete elimination of all batteries forsupplying operating potentials to the system. As is also true ofthearrangement of Fig. 6, the automatic volume control not only compensatesfor fluctuations in the strength of the incoming signals. but alsocompensates for reasonable variations in line voltage of the alternatingcurrent powertsupply. p

As in Fig. 6 the variable timing condensers (i1-C, are grounded in orderto eliminate undesirable capacity effects as well as to make itpracticable to connect the condensers on a single shaft for uni-control,if desired. As in Fig. 6, it will be seen that the power supply of Fig.'7 is not grounded. thus eliminating the danger of short-circuiting thedirect current supply when a separate ground is necessary for thealternating-current rectifying and filtering system. In certain.instances, Figs. 6 and 7 differ in the connection of certain by-passcondensers. The purposes and reasons for the positions of these by-passcondensers should be apparent to those skilled in the art.

Assuming that the vacuum tubes employed are of the type having five-voltfilaments, volts of filament, or A, supply is needed. As abovementioned, the automatic volume control is here applied to two tubes,namely 58 and 60; the cut-ofiI being effected with the use of twodifferent plate, or B, voltages. The plate electrode 66 of rectifier 64is more negative relative to the filament of tube 58 than relative tothe filament of tube 60, by the 5-volt drop across one filament. Tocompensate for this difference, volts higher B voltage is applied to theplate of tube 58 than to the plate of tube 60, which makes both tubeseut off vpractically at the same time. The reason for applying a Bvoltage of 85 volts (which. of course, is to be added to the 35 volts Avoltage) tothe plate of tube 60, whereas a B voltage of 90 volts isapplied to the plates of the radio-frequency amplifying tubes of Fig. 6,is that the arrangement of Fig. 7 employs one more tube than Fig. 6; thedifference of 5 volts being included in the A supply, as will be seen ycomparing the two figures. Thus, actually, the plates of theradio-frequency amplifying tubes 119, 120 and 121 of Fig. 6 are providedwith 90, 95 and 100 volts, respectively. Similarly the plates of theamplifying tubes of Fig. 7 are supplied with 85 to 155 volts.

In addition to the combined advantages just outlined, the arrangement ofFig. 7 also includes an additional feature which has not previously beendescribed, namely, the means 130 for determining the filament currentsupplied to one of the amplifying tubes. As has been explained inconnection with Fig. 6,-when operating the filaments of the severalvacuum tubes on rectified and filtered current from an alternatingcurrent power source, it is desirable that the filaments be connected inseries since it is at present more practicable to provide a currentsupply at a comparatively high voltage and lowv current. Fig. 6 shows ashunt rheostat 129 connected in parallel with the filament of tube 125so that the current divides between the rheostat and the filament.iVhile this means for controlling the filament emission of a singletube, as shown in Fig. 6, is fairly satisfactory, the arrangement,shownin Fig. is a substantial improvement. With the former method, anincrease in current throughthe controlled filament is accompanied by asmaller increase in current through the other filaments in series. Theimproved arranvement shown in Fig. 7, on the other hand, providing threeresistances, two of which are simultaneously variable, allows avariation of the voltage on one or more filaments without affecting thecurrent through the other filaments; or, more generally, withoutchanging the load on the filament-current supply. It is apparent thatthe benefits of this device will be especially manifest in anarrangement such as the present, wherein the current supply for thefilaments is obtained from a rectified, filtered alternating currentsource, particularly when the rectifying device is of the common typewithout automatic voltage regulation. The compound rheostat 130comprisingresistances R1 and R2 is so arranged that a movement of thecontrol knob Will increase the one resistance, while diminishing theother in proportion. ne of these resistances, namely R1 is connected inseries with resistance R0 in shunt with filament 74 of tube 7l, theresistance of filament 74 being represented by Rf It may here be pointedout that While Fig. 7 illustrates the manual control of the filament ofonly one tube, namely 74, the filaments of other tubes could beconnected either in series or parallel with filament 74 if it weredesired that independent simultaneous control be had of more than onefilament. Rf may, therefore, be taken to represent the effectiveresistance of the filaments to be controlled, When the two resistancesR1 and R2 and the fixed resistance 4resent the Ro are properlyproportioned to the normal gr operating reslstance Rf of the filament orfilaments of the tubes to be controlled,the resistance of the system asa whole will remain substantially constant during adjustment of thecontrol device 130. By way of illustration, the following data are givenfor Fig. 7, assuming the tubes to be all of the well-known 201A type, of20 ohms resistance: It, will equal 20 ohms; RQ may equal Rf; R1 mayequal 8Rf; and R2 may equal l/zRf. Accordingly, to control one tube whenRf equal 20 ohms, R1 R2, 10 ohms.

It is believed unnecessary to describe the method of controlling thesignal amplification in the arrangementsof Figs. 5, 6 and 7 since theyare substantially similar in operation to that of the systems describedin reference to Figs. 1, 3 and 4. It should be mentioned, however, thatthe advantages of the present invention are especially apparent insystems such as shown in Figs. 6 and 7, because of the fact that anyreasonable fluctuations in the voltage of the power supply line are thusautomatically compensated for, and uniform volume of signals is assured.

There are advantages attending the use, in connection with the presentinvention, of the two-electrode rectifier circuit typified by Figs. 1, 3and 7, which may not be apparent from the foregoing discussion. It isimpossible to overload this type of rectifier, and the rectified outputvoltage is directly proportional to the applied alternating signalvoltage when this voltage is large, say over two volts. The controlsystem in the circuits of the figures referred to requires a largeoperating voltage, say ten Volts, so that the latter condition of largesignal voltage is realized. No such simple relationship is possible in athree-electrode detector, whose rectified output never exceeds alimiting upper value, and is never proportional to the applied voltage,except over a very small range of voltages. This distinction will beseen from Fig. 8 where the abscissae A. C. repalternating sionalvoltages, whereas the ordinates D. C. represent the rectified outputvoltages. It is well known that the linear curve is much more desirablewhen minimum distortion of a modulated signal is desired, and it will beobserved from will equal 160 ohms, and

' Fig. 8 that the preferred type of curve is obbe construed as allmitatlon, but merely as tained from the two-electrode rectifier.

A further advantage of the linear type detector with the automaticvolume control connection and a visual resonance indicator in the anodecircuit of the amplifier whose grid bias is being automaticallycontrolled, lies in the fact that the visual resonance indicator willgive an indication which is proportionate to the received signalintensity. This follows from the fact that the negative each filamentbeing equals 2O ohms, R0 willl id bias on the amplifier is directlyproportional to the strength of the signal; and hence the anode currentbears a similar relation to the signal.

The three-electrode detector is useful for relatively small appliedvoltages, and the rectified output voltage is then approximatelyproportional to the square of the applied voltage, i. e., to the powerassociated with thel applied Voltage. For this reason the rectifiedvoltage increases with the carrier wave modulation. When such a detectoris used in the control system, as in Figs. 4.-, 5 and 6, the total powerfrom the radio-frequency amplifier is maintained at a substantiallyconstant level, the amplitude of the carrier wave being decreased in thepresence of modulation. It is desirable to maintain vthe carrier wave atal constant amplitude at the output of the amplifier, and this isaccomplished by the two-electrode rectifier as shown in Figs. 1, 3 and7. The control system maintains constant the average signal amplitudewhich is equal to the carrier wave amplitude and independent of thedegree of modulation.

It will be observed that in a system employing a two-electrode rectifiersuch as represented by valve 33 of Fig'. 1, and 64. of Figs. 3 and 7,the control bias voltage is independent ofthe B or anode vbatteryvoltage. Since the rectifier is not an amplifier, is not critical, andrequires neither anode nor bias: ing battery, no adjusting devices arerequired. This is not the case in the three-electrode detector circuits,so that a potentiometer, 98 or 128 in Figs. 4 or 6, respectively, mustbe adjusted as described to accommodate the control bias to anyparticular combination of tubes and B voltage. On the other hand, thelatter type of detector is more sensitive because it is also anamplifier, so that the control system operates on aA smaller appliedalternating voltage.

In the foregoing description, tuned radiofrequency receivers of theneutralized and unneutralized types have been referred to. It should bepointed out however, that the present invention may be employed withequaleffectiveness -to any radio receivers in wired radio and spaceradio systems, and that it has been found especially applicable. toreceivers of the super-heterodyne type. For this reason the presentdisclosure of typical embodiments of the invention should notillustrative of the principles' of the invention, the scope of which isdefined in the appended claims.

What is claimed is:

1. In a signaling system a vacu-um tube amplifier having a cathode and acontrol electrode, a vacuum tube detector coupled to said amplifier,said detector having an output electrode, means-for maintaining saidoutnevases put electrode normally negative relative to at least part ofsaid amplifier cathode, means for causing said output electrode tobecome more negative in the presence of an amplified signal, and adirect-current connection between said control electrode and said outputelectrode, whereby the amplification or said amplifier is regulatedautomatically.

2. In a carrier-current signaling system, in combination, a vacuum tubeamplifier having a cathode and a control electrode, a vacuum tubedetector coupled directly to the output of said amplifier, said detectorhaving'y a cathode and an output electrode, means for maintaining saiddetector cathode at substantially the same potential as said amplifiercathode, means for maintaining said output electrode at a negativepotential with respect to said cathodes, means causing said outputelectrode to become more negative in the presence of an amplifiedsignal, and a directcurrent connection between said output electrode andsaid control electrode.

3. A combination according to claim 2 in which the means for maintainingthe detector output electrode at a negative potential with respect tosaid cathodes is a resistance connected between said output electrodeand the detector cathode.

4. In a modulated carrier-current signalling system employing acarrier-current amplifier and rectifier, which rectifier produces amodulated uni-directional voltage, a direct-current connection from saidrectifier to an element of said am lifier whereby the amplification isregulated) automatically, and a connection from said rectifier to amodulation current amplifier whereby the signal is further amplified,said connection from said rectifier to said modulation current amplifierincluding a condenser in series for preventing the uni-directionalcomponent from being impressed upon the input of said modulation currentamplifier.

In a signaling system, a vacuum tube amplifier having a cathode and acontrol electrode, a detector coupled to said amplifier, said detectorhaving an output electrode, means for maintaining said output electrodenormally negative relative to at least part of said amplifier cathode,means for causing said output electrode to become more negative in thepresence of an amplified signal, and a direct-current connection betweensaid control electrode and said output electrode, whereby theamplification of said amplifier is regulated automatically.

6. In a signaling system a vacuum tube amplifier having a cathode and acontrol electrode, a second vacuum tube having an out` put electrode,means for coupling the output of said amplifier with said second tube,means for maintaining said output electrode normally slightly negativerelative to at least part of said cathode, means for causing said forcausing the said output electrode to become more negative in thepresence of an amplified signal, a tuning arrangement for tuning saidamplifier to a desired signal, a direct current connection between saidcontrol electrode and said output electrode, whereby the amplificationof said amplifier is regulated automatically, and means for visuallyindicating the condition of resonance 1n saidtuning arrangement, wherebytuning is facilitated.

8. An arrangement according to claim 7 in which said tuning arrangementand said means for visually indicating the condition of resonance areconnected in the anode circuit of said amplifier.

9. In a signaling system a vacuum tube amplifier having a cathode and acontrol electrode, a vacuum tube detector coupled to the output ofsaidamplifier, said detector having a cathode and an output electrode, meansfor maintaining said detector cathode at a potential greatly negativerelative to said amplifier cathode, means for maintaining said outputelectrode at a potential normally slightly negative relative to saidamplifier cathode, means for causing said output electrode to becomemore negative in the presence of an amplified signal, anda-direct-current connection between said output electrode and saidcontrol electrode whereby the amplification of said amplifier isregulated automatically. l0. In a signaling system, a vacuum tubeamplifier having a cathode and a control electrode, a diode detectorcoupled to said amplifier, said detector having an anode, means formaintaining said anode normally negative relative to at least part ofsaid amplier cathode, means for causing said anode to become morenegative in the presence of an amplified signal, and a direct-currentconnection between said control electrode and said anode, whereby theamplification of said amplifier is regulated automatically.

1l. In a signaling system, a vacuum tube amplifier having a cathode anda control electrode, a diode detector coupled to said amplifier, saiddetector having a cathode and an anode, means for maintaining saidcathodes at substantially the same potential, means including a highresistance connected between the detector anode and cathode formaintaining said anode normally slightly nega-tive relative to saidcathodes, means for causing said anode to become more negative in thepresence of an-amplied signal, and

a direct-current connection between said control electrode and saidanode, whereby the amplification of said amplier is regulatedautomatically. 12. In a signaling system a vacuum tube amplifier havinga cathode and a control electrode, a second vacuum tube fhaving anoutput electrode, means for coupling said amplifier with said secondtube, means for maintaining said output electrode normally slightlynegative relative to at least part of said cathode, means for causingsaid output electrode to become more negative in the presence of anamplified signal, a tuning arrangement for tuning said 'amplifier to adesired signal, and a direct-current connection between said controlelectrode and said output electrode, whereby the amplification of saidamplifier is regulated automatically, and means for visually indicatingthe condition of resonance insaid tuning arrangement whereby tuning isfacilitated.

In testimony whereof I afx my signature.

HAROLD A. WHEELER.

